A storage system improves its performance in terms of response to a host computer by installing a cache memory capable of reading/writing at high speed data that is requested by the host computer to be read or written. As the cache memory, which is required to read/write data at high speed, a volatile DRAM is usually used.
The cache memory is a memory for temporarily storing data that is requested by the host computer to be written, and consequently is small in capacity and not so large in power consumption. Therefore, in the event of a power outage, a battery supplies power to the cache memory to keep dirty data which is not stored in a disk drive but is stored in the cache memory. This means that the data can be kept only for a limited period of time that is determined by the capacity of the battery.
In recent years, cache memory capacity is increasing as the storage capacity of storage systems becomes larger. Cache memories consume accordingly more power. The enhanced speed of cache memories is another factor that increases the power consumption of cache memories.
The increase in the power consumption of cache memories shortens the period of time in which a cache memory can keep stored dirty data with the use of a battery. As a countermeasure, a backup method for keeping data stored in a cache memory during a power outage has been introduced in which, when a power outage occurs, the electric power of a battery is used to migrate data stored in a cache memory to a non-volatile flash memory (see JP 2009-237881 A).